The present disclosure relates generally to light emitting systems and methods for controlling nanocrystal distribution therein.
Hybrid light emitting systems combine inorganic nanocrystals and organic molecules. Such systems often include the organic polymer with the inorganic nanocrystals incorporated therein. The organic polymer/inorganic nanocrystal layer may be one of many stacked layers in such systems. Some light generation mechanisms are more efficient when the nanocrystals are present at specific location(s) within the layer. As an example, one mechanism may be more efficient when the nanocrystals are placed in the vicinity of one or both of organic excitons or exciplexes, while another mechanism may be more efficient when the nanocrystals are uniformly dispersed within the interfacial plane separating two organic layers. Inorganic nanocrystal and organic polymer systems tend to minimize their energy by redistributing the nanocrystals in a manner favorable for thermodynamic stability and potentially deleterious to hybrid device performance. As a result, nanocrystals tend to phase-segregate to the surfaces of the polymer layer, to form large aggregates within the polymer layer, or combinations thereof. Redistribution of nanocrystals after phase segregation or aggregation may not only reduce their ability to emit light, but it may also, in some instances, degrade the quality of the polymer.
Further, the polymer layer surface in light emitting systems often forms an interface with a metal established adjacent thereto. Segregation of nanocrystals to the polymer-metal interface may result in a parasitic energy loss mechanism, where the excited nanocrystal transfers its energy to the metal polarons before its radiative relaxation (and light emission) occurs. Nanocrystals at the polymer layer surface may also be removed or damaged as the subsequent layer (e.g., metal) is established on the polymer.
Attempts to reduce nanocrystal phase segregation include controlled removal of solvent from the polymer/nanocrystal layer, inhibiting nanocrystal movement via polymer structure, incorporating nanocrystal ligands that may improve the miscibility of the nanocrystals within the polymer layer, altering processing conditions, and forming chemical bonds between the nanocrystals and the polymers. The application and effectiveness of such techniques may undesirably be limited to specific nanocrystal concentrations and/or the polymer-nanocrystal combination.